Recently, an illumination device using point light source, such as LED illumination device, has been used as diversifying light sources for indoor lighting fixtures including bulb lighting and fluorescent lighting and backlight of LCD televisions. The illumination device using point light source is also used for illuminating a subject at taking a picture with an inserted camera of a mobile phone, digital still camera (DSC), or camcorder. In addition to downsizing, higher lighting intensity close to an illumination device equipped with flashlight discharge tube, which is a conventional light source, is demanded for an LED illumination device built in DSC or camcorder that is downsized year by year.
A structure of the illumination device equipped with conventional LED element is described below with reference to FIG. 4. FIG. 4 is a sectional view of the conventional LED illumination device.
As shown in FIG. 4, conventional LED illumination device 27 includes multiple LED packages 21, printed circuit board 26, housing 25, and lens 23. (For example, see PTL 1.) Multiple LED packages 21 are attached to printed circuit board 26. Printed circuit board 26 is attached to the bottom of housing 25. Lens 23 for collecting light from LED packages 21 is attached to an upper part of housing 25. However, since printed circuit board 26 and lens 23 are attached via housing 25 in LED illumination device 27 of PTL 1, downsizing is difficult.
On the other hand, in a downsized LED illumination device, it is important to collect light to an illuminating range as much as possible and illuminate a subject with uniform light distribution. This increases the lighting intensity and achieves clear shooting.
However, to realize high lighting intensity or uniform light distribution, positional accuracy becomes necessary between components configuring the LED illumination device. Accordingly, requirement for positional accuracy of LED packages 21 and lens 23 becomes tough due to downsizing. The above LED illumination device to which LED packages 21 and lens 23 are attached via housing 25 has disadvantage with respect to accuracy because factors of dimensional variations increase. As a result, the above LED illumination device likely causes variations in optical characteristics, such as lighting intensity and light-distribution angle.
LED illumination device 39 that reduces variations in optical characteristics caused by positional accuracy is described with reference to FIG. 5. (For example, see PTL 2.) FIG. 5 is a sectional view of another conventional LED illumination device.
As shown in FIG. 5, conventional LED illumination device 39 includes LED package 31 including LED chip 33 that is a light source, base substrate 32, and transparent resin 38; and lens 34. LED chip 33 is attached to a top face of the bottom of concavity 32B in base substrate 32. Transparent resin 38 is configured by mixing wavelength-converting phosphor for converting a wavelength of light emitted from LED chip 33 and transparent resin, such as epoxy resin and silicone resin. LED package 31 is configured by filling transparent resin 38 into concavity 32B in base substrate 32 to seal LED chip 33.
Lens 34 is attached to an upper part of LED package 31 within a luminous area of LED package 31, and collects light emitted from LED chip 33.
Here, lens 34 is directly attached such that its positioning face 34A at the bottom edge of lens 34 touches reference face 32A at the top face around concavity 32B in base substrate 32 of LED package 31.
On the other hand, another method of increasing the lighting intensity in LED illumination device 39 is to increase the lighting current. However, it generates heat. In general, LED chip 33 has a characteristic that lower temperature results in higher efficiency of converting the current to light (luminous efficiency). Therefore, the luminous efficiency of LED chip 33 reduces and also operating life shortens as a temperature of LED chip 33 rises by increasing the lighting current. Furthermore, transparent resin 38 sealing LED chip 33 also discolors by thermal impact, and the light transmittance reduces. This reduces the lighting intensity of LED illumination device 39.
Therefore, high heat-release characteristics for reducing heat is required for LED illumination device 39 in order to prevent an impact on optical characteristics of the heat generated by illuminating LED chip 33.
However, in LED illumination device 39 shown in FIG. 5, lens 34 is directly attached to reference face 32A of base substrate 32 of LED package 31. Therefore, the entire incoming face of light of lens 34 touches base substrate 32 and transparent resin 38 of LED package 31. The heat generated from LED chip 33 is thus easily transferred to lens 34 via base substrate 32 and transparent resin 38. Here, if a current level applied to LED chip 33 is increased in order to increase the intensity of light from LED chip 33, a heat quantity generated from LED chip 33 also increases proportionately. A heat quantity transferred to lens 34 thus also increases proportionately. As a result, lens 34 thermally expands due to an impact of heat. For example, optical characteristics change by a minute change of shape. In addition, a change of shape of lens 34 deteriorates optical characteristics, such as reduction of lighting intensity and a change of light-distribution angle.
Still more, luminous efficiency of LED chip 33 in LED package 31 decreases as a temperature increases. Therefore, an amount of light reduces due to decreased luminous efficiency if LED chip 33 is used at high temperature for a long period. If the current level is further increased to gain necessary lighting intensity for LED illumination device 39, power consumption increases.
Furthermore, if LED chip 33 is continuously used at high temperature, thermal discoloration of transparent resin 8 used for sealing accelerates. As a result, transmittance of lens 34 reduces, and thus the lighting intensity of LED illumination device 39 reduces. This shortens the operating life.
LED illumination device 27 of PTL 1 described with reference to FIG. 4 has a structure unlikely affected by heat. However, optical characteristics, such as lighting intensity and light-distribution angle, may vary.